Method and device for laser drilling in a process gas atmosphere

ABSTRACT

A method and device for laser drilling in which the geometric form of the drill-hole wall is influenced by reciprocal action between a laser beam and a supplied process gas, which thereby is ionized to plasma. Furthermore, the outlet opening of the drill hole is influenced by a suitable arrangement of a backing.

BACKGROUND INFORMATION

Precision micro-holes in nozzles of fuel-injection systems are usuallyintroduced with the aid of erosion methods. Using this technology, it iscurrently possible to produce minimal diameters of approximately 120 μmin large-scale production. Furthermore, laser drilling also allows theproduction of precision holes having diameters of less than 120 μm, butthis has not yet been introduced as large-scale method.

In the fuel-injection field, there is increasing demand for conicalholes to the effect that a fuel-outlet orifice have a smaller diameterthan a fuel-intake orifice. Such precision micro-holes are alreadyutilized in systems for diesel fuels (direct injection) or for gasoline(manifold and direct injection).

German Patent No. DE 199 055 71 describes a laser-drilling method inwhich a laser beam executes a wobbling motion relative to a workpiece.This has the effect that a cone-shaped shell surface is traversed insidethe workpiece. The polarization plane of the laser beam is rotated insynchrony with the wobbling motion.

German Patent Application No. DE 100 548 53 describes a method forintroducing a micro-hole in a workpiece by means of a laser beam. Inthis case, the focus of the laser beam is moved continually along acircular path which is concentric with respect to the hole axis, thelaser beam being composed of a succession of short laser pulses.

In all current drilling methods, the produced bore must be subjected tofollow-up treatment by hydro-erosive (HE) rounding. In fuel injectionsystems, for instance, this is done primarily to round the edge of thefuel intake (in addition to improving the surface of the bore wall andreducing the variance among the hydraulic flow rates of the individualbore holes of a nozzle). This results in a considerable reduction of theflow resistance at this point and also reduces undesired cavitationmanifestations.

An additional considerable improvement in this direction is achieved bythe combination of conicalness and distinct HE-rounding.

SUMMARY OF THE INVENTION

The present invention is intended to selectively influence the profileof the outlet of the bore hole (fuel intake) during the drillingoperation using short-pulse (ns) or ultra-short pulse (ps/fs) lasers. Inparticular, a simple and low-cost production of various symmetrical andasymmetrical outlet profiles is provided, such profiles including, forinstance, widened regions and bulges.

In the laser drilling method provided according to the presentinvention, a region of a component is acted upon by a laser beam and ahole is produced within this region. This action or drilling isimplemented under an adjustable process-gas atmosphere. Due toreciprocal action between the utilized laser beam and the selectedprocess gas within the area or hole acted upon by the laser beam, plasmais generated by ionization of the process gas. According to the presentinvention, it is additionally provided that a backing be arranged at anoutlet orifice of the hole produced by the laser beam.

The process gas or gas mixture utilized according to the presentinvention primarily serves to increase the processing quality and tooptimize the processing time, in particular to shorten the processingtime. By creating special process-gas atmospheres, the laser beam -material interaction is indirectly influenced in the impinged-uponregion or hole, and thus the processing process as well. The type ofprocess-gas atmosphere determines the properties of the plasmas thatforms according to the present invention during the reciprocal action ofthe utilized laser radiation with the material or matter to beprocessed, the plasma formation being aided by material vapor. Asolid-state laser (Nd:YAG), for instance, is used as a laser. Differentprocess gases in each case produce plasmas that may differ in theirtemperature and expansion, for instance.

To be mentioned as a particular advantage of the method according to thepresent invention is the possibility of considerably reducing theprocess time of the HE-rounding to produce rounded forms, or of makingthe HE rounding entirely unnecessary to begin with. Furthermore, usingthe method according to the present invention, the fuel intakes may bedesigned in a way that is not possible with HE-rounding. According tothe present invention, it is possible, in particular, to introduceasymmetrical rounded forms or symmetrical or asymmetrical bulges rightbehind the fuel intake. Rounded forms have the advantage of an improvedintake response, whereas bulges may be used to prevent, or alsoselectively generate, turbulence that may occur at the fuel intake.

Particularly important for the selective design of the outlet is theplasma state shortly before and during the reemergence of the laser beamfrom the material. Depending on the selected parameters such as gascomposition, gas pressure and/or gas flow direction, rounded forms,bulges and/or sharp discharge edges may form; these phenomena may occuracross the entire bore-hole outlet in an even, rotationally symmetricalmanner, or they may come about on one side only.

Rear-space protection materials, so-called backings, are used to preventthe free propagation of the laser beam after it reemerges from thematerial or matter. This may normally be utilized to prevent damage toanother workpiece or component, or areas of the same workpiece.Furthermore, such backings are able to maintain the state of a closedbore hole for a certain period of time and thus contribute to theproduction of a desired discharge orifice or a bore hole outlet.

Depending on the application, backings are made from various materialssuch as polymers, metals or ceramic materials.

A backing is able to influence the formation of a bore hole outlet byreflecting the arriving laser radiation back in the material directionor in that it influences the plasma, ionized from the process gas, inits propagation, thereby leading to additional material removal. Anadditional effect may come from a plasma that is possibly produced byerosion of the backing.

For practical purposes, the method uses as process gas an inert gas suchas nitrogen, in particular with the addition of noble gases such ashelium, argon and the like. The use of such a process gas has theadvantage that an area to be acted upon is rendered inert, therebypreventing oxidation of this region. Moreover, such a composition of theprocess gas ensures adequate surface qualities of the bore hole wall andmelt-film thicknesses.

Furthermore, the process gas is preferably pressurized, the pressurebeing preferably set to below approximately 1.5 bar. This promotes theformation of hollow forms in the produced holes. It is also possible toselect higher pressures, which allows hollow forms to be suppressed.Overall it may be said that the pressures utilized according to thepresent invention result in hollow forms in the vicinity of the edgeregion of the workpiece to be processed. The higher the pressureutilized, the greater the shift of a hollow form toward the interior ofthe workpiece.

Furthermore, it may preferably be provided to set the impingementdirection of the process gas by tilting relative to the direction of thelaser beam, the tilting angle possibly amounting to between 0° und 15°.Suitable selection of a tilting angle ensures that hole shapes, inparticular hollow forms or widened forms, with various degrees ofasymmetry are able to be formed.

Due to the plasmas produced in implementing the method according to thepresent invention, calculated pressures on the order of a few 100 barand flow velocities of several 10 km/s may occur in the acted-uponregion or hole. Due to an accelerated removal of a melt resultingtherefrom, among others, an active contribution is rendered towardshigher material removal.

Apart from the prevailing introduction conditions of the process gas(composition, pressure, direction), the ionization of the process gasmay also be influenced by, in particular, the properties of the laserbeam such as wavelength and output.

A backing used in this context may exhibit thermal or opticalcharacteristics that influence the shape or the design of the outletorifice. Suitable metallic materials, especially copper, are providedhere, in particular. These material properties are especially importantfor the degree of the widening of the outlet orifice. For example, whenusing laser wavelengths of 1064 μm and a backing of copper which isarranged at a suitable distance from the outlet orifice, relativelylarge widened forms are produced, whereas relatively small widened formscome about with a backing of steel. Furthermore, the geometry of thebacking may influence the form or design of the outlet orifice. In thepresent invention, the use of copper as backing material is particularlypreferred. It should be mentioned that copper is generally not favoredin engine building since sulfide may form in the case of copper depositsin the engine area due to reciprocal actions with sulfide that ispresent in the fuel, such sulfide formation having a negative influenceon the service life of the engine. However, these disadvantages areclearly outweighed by the properties of copper that are able to beutilized within the framework of the present invention in connectionwith the production of specially formed holes in workpieces.

Furthermore, it may preferably be provided to arrange the backing at adistance from the outlet opening that influences the shape of the outletorifice. Such a distance is preferably selected between 20 μm to 200 μm.

By appropriate selection of such a distance, the geometry of the widenedform of the borehole exit may be influenced in a simple manner.

The backing may be arranged at a specified angle by tilting relative tothe outlet orifice. Various degrees of tilting produce widened areas ofthe outlet hole of the borehole whose asymmetries vary accordingly. Theangles preferably used for this purpose are in the range of 0° to 20°.

Especially by suitable selection of the form of the backing or thearrangement of the backing relative to the outlet orifice, symmetricalor asymmetrical rounded forms are able to be introduced in a region ofthe outlet orifice of a hole in an uncomplicated manner, yielding theadvantage of no longer requiring retroactive HE rounding at all orrequiring such rounding only to a limited degree.

The device according to the present invention by which the method of thepresent invention, in particular, is able to be implemented, isdistinguished by the provision of a laser beam, a retaining device for acomponent to be processed and means for adjusting a process-gasatmosphere.

At least one gas nozzle may preferably be provided as means foradjusting the process-gas atmosphere. In this way, flowing process gasof a suitable composition may be aimed at the region of a workpiece tobe acted upon in a manner that is easy to control or regulate and may becarried out at a suitable pressure and an appropriate angle. A suitablecomposition of the process gas may be provided by a gas mixer.

According to the present invention, the device may additionally berefined in such a way that a backing is able to be positioned at anoutlet orifice produced in the component by the action of the laserbeam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of a preferred specificembodiment of the device according to the present invention forimplementing the laser-drilling method according to the presentinvention.

FIG. 2 shows a detail of the device according to FIG. 1, in an enlargedrepresentation.

FIGS. 3 a through FIG. 3 e show schematic representations of boreholesproduced according to the present invention, in a lateral sectionalview.

FIGS. 4 a through 4 c show schematic representations of boreholesproduced according to the present invention, in a lateral sectional viewor in a plan view on the basis of electron-microscopic photographs.

DETAILED DESCRIPTION

The illustrated specific embodiment of the device according to thepresent invention is designated as a whole by 100. A laser beam 30generated by a laser 31 first traverses an expanding lens 32. Laser beam30 is partially reflected or switched at a shutter 34, scattered orexcess residual radiation being absorbed by a radiation sink 33. Thedirection of laser beam 30 inside its optical path is deflected by oneor a plurality of mirrors 37. A trepanning lens 35 is also disposedinside the optical path, and the device for focusing the laser beam alsohas a focusing lens 36. In addition, the optical path of laser beam 30runs through a gas nozzle 12 and impinges upon a workpiece 40 to beprocessed.

The process gas required to implement the method according to thepresent invention is provided by a gas mixer 11 and forwarded to a gasnozzle 12 via a line 11 a. The process gas is blown directly ontoworkpiece 40 with the aid of a gas nozzle 12.

Workpiece 40 is mounted in a handling device 50. Using suitablemeasures, this handling device 50 is moveable in all three spatialdirections x, y and z, so that a suitable position of workpiece 40 forimplementing the method is able to be adjusted. Positioning the focusinglens 36 along axis z allows laser beam 30 to be varied in its focusingposition. It is likewise possible to position gas nozzle 12 with respectto the direction of the optical path of laser beam 30 running throughgas nozzle 12, or to position it relative to workpiece 40. In theprocess, a movement along direction x or y takes place. A rotation ofgas nozzle 12 is also able to be realized by means of a suitablemechanism, such rotatability being symbolized by φ in FIG. 1.

The method for laser drilling according to the present invention is ableto be realized in a very simple manner when using this specificembodiment of device 100 according to the present invention. Laser beam30, its optical path having traversed gas nozzle 12, impinges uponworkpiece 40 within a region to be acted upon. A selected or adjustedprocess gas, provided by gas mixer 11, flows under pressure from anadjustable direction out of nozzle 12 onto the region of workpiece 40 tobe acted upon. This, in particular, promotes the ionization of processgas to plasma by reciprocal action between process gas and laser beam 30within the acted-upon region.

FIG. 2 shows a cut-away portion of FIG. 1. Using a mounting support 51workpiece 40 is able to be firmly positioned with respect to handlingdevice 50, a suitable position of workpiece 40 relative to handlingdevice 50 being adjustable by means of movement via mounting support 51.In the same way, a backing 20 is able to be firmly positioned relativeto handling device 50 via a holding device 52, a suitable position ofbacking 20 relative to handling device 50 being adjustable via holdingdevice 52 by movement. By controlling mounting support 51 or 52, backing20 and workpiece 40 may be spatially positioned as desired with respectto each other; this may be done both before and also during thedescribed method.

A hole 44 to be produced in workpiece 40 by means of the laser-drillingmethod is created in the location of workpiece 40 where laser beam 30impinges upon workpiece 40 or acts upon it, the laser beam exiting at anoutlet opening 43 of hole 44 on the side of workpiece 40 that facesbacking 20. The process gas, which is blown by a gas nozzle (not shown)onto workpiece 40 at a suitable angle and from an appropriate distance,is ionized to plasma by reciprocal action with laser beam 30 in theregion of hole 44 or a region of outlet opening 43.

This produces selective geometrical forms in the region of hole 44 oroutlet opening 43 as will now be illustrated by way of example with theaid of the following figures.

FIGS. 3 a through 3 e show detailed views of holes or bore holes 44within workpiece 40 in a sectional view in parallel with a bore holecenter axis 46 of hole 44. A laser beam 30 (not shown) had been aimed atworkpiece 40 from the left, in parallel with bore-hole center axis 46;laser beam 30 penetrated workpiece 40 in the impingement region, theentrance location not being shown in FIGS. 3 a to 3 e. The individualoutlet openings 43 of holes 44 are visible in FIGS. 3 a to 3 e.

In FIG. 3 a, a region of outlet opening 43 of bore hole 44 has asymmetrical widened form 41 or rounded form, which is the result of apreferably symmetrical arrangement of a backing (not shown) with respectto bore-hole center axis 46 within a region behind—in the illustrationof FIGS. 3 a to 3 e, to the right—of outlet opening 43. The degree ofthis widened form 41 or rounding is able to be influenced by appropriatespacing of the backing relative to outlet opening 43.

Widened form 41 of an area of outlet opening 43 of bore hole 44 shown inFIG. 3 b has an asymmetrical design or is rounded on one side only. Inthis example the method according to the present invention wasimplemented in such a way that the backing was arranged in a regionbehind outlet opening 43 and tilted at a suitable angle with respect tobore hole center axis 46.

Bore hole 44 from FIG. 3 c has symmetrical or bilateral bulges 42 in aregion of outlet opening 43 of hole 44. Such a bulge 42 is realizable bythe described reciprocal action between laser beam and formed plasma,for example.

Bore hole 44 shown in FIG. 3 d has an asymmetrical bulge 42. Such anasymmetrical form of bulge 42 is facilitated by tilting of a nozzle thatintroduces the process gas, such tilting being implemented at a suitableangle relative to bore hole center axis 46 or the laser beam.

Bore hole 44 in FIG. 3 e is designed such that it has a combination ofwidened form 41 and bulge 42 in a region of outlet opening 43. Such adesign may be realized by combining the afore-described measures(tilting of the backing and impingement of the process gas at an angle).

Additional specific embodiments of holes 44 or outlet openings 43 withinworkpiece 40 produced by the method according to the present inventionare shown in FIGS. 4 a to 4 c in longitudinal section or in a plan view.

FIG. 4 a shows two different forms of bulges 42 in longitudinal section.

Furthermore, intake openings 45 are shown, which are produced byimpingement of the workpiece by a laser beam arriving from the left, andhave a smaller diameter than outlet openings 43 on the right. In hole44a shown above, bulge 42 is asymmetrical, analogously to FIG. 3 d,whereas two symmetrical bulges 42 are shown in lower hole 44 b,analogously to FIG. 3 c.

Outlet opening 43 illustrated in FIG. 4 b has an asymmetrical widenedform 41 or rounding. This is realized by appropriate measures as theyhave been described with reference to FIG. 3 b. FIG. 4 c shows an outletopening 43 from the same perspective as FIG. 4 b, this opening having asymmetrical widened form 41 or rounding, which is realized by analogousimplementation of the method according to the present invention as itwas described with reference to FIG. 3 a.

1-9. (canceled)
 10. A method for laser drilling comprising: acting upon a region of a workpiece by a laser beam, so that a hole is produced in the region; implementing the method under an adjustable process-gas atmosphere in such a way that, due to a reciprocal action between the laser beam and a process gas, plasma forms in at least one of the region and the hole acted upon by the laser beam; and arranging a backing at an outlet opening of the hole produced by the laser beam.
 11. The method according to claim 10, wherein an inert gas, with an addition of noble gases, is used as the process gas.
 12. The method according to claim 11, wherein the inert gas includes nitrogen, and the noble gases include at least one of helium and argon.
 13. The method according to claim 10, further comprising pressurizing the process gas at a pressure of maximally 1.5 bar.
 14. The method according to claim 10, further comprising adjusting an impingement direction of the process gas by tilting relative to a direction of the laser beam, a tilting angle being up to 15°.
 15. The method according to claim 10, wherein a material used for the backing has at least one of thermal and optical properties that influence a form of the outlet opening, the material including at least one of a metallic material and a copper-containing material.
 16. The method according to claim 10, wherein the backing is arranged at a distance from at least one of the outlet opening and the workpiece that influences a form of the outlet opening, the distance being between 20 μm and 200 μm.
 17. The method according to claim 10, wherein the backing is arranged with tilting at a predetermined angle with respect to at least one of the outlet opening and the workpiece, the tilting influencing a form of the outlet opening, the tilting angle being up to 20°.
 18. A device for laser drilling comprising: means for acting upon a region of a workpiece by a laser beam so as to produce a hole; and means for adjusting a process-gas atmosphere in at least one of the region and the hole acted upon by the laser beam, in such a way that, due to a reciprocal action between the laser beam and a process gas, plasma forms in at least one of the region and the hole acted upon by the laser beam, a backing being able to be positioned at an outlet opening of the produced hole.
 19. The device according to claim 18, wherein the means for adjusting the process-gas atmosphere includes at least one gas nozzle. 